This invention relates to imaging systems, and more particularly to improved development systems.
The formation and development of images of photoconductive material by electrostatic means is well known. The basic xerographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material refered to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in the image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or over coating treatment may be substituted for the foregoing heat fixing step.
Similar methods are known for applying electroscopic particles to electrostatic latent images to be developed. Included within this group are the "cascade" development technique disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, the "magnetic brush" process disclosed in U.S. Pat. No. 2,874,063, and the "powder cloud" process disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776, the disclosures of which are hereby incorporated by reference.
An additional dry development system and the dry system to which this invention is most nearly directed involves developing an electrostatic latent image with a powder developer material, the powder having been uniformly applied to the surface of the powder applicator. The latent image is brought close enough to the developer powder applicator so that the developer powder is pulled from the applicator to the charge bearing surface in image configuration. The image and powder applicator may desirably be brought in contact including contact under pressure to effect development. The powder applicator may be either smooth surfaced or patterned so that the developer powder is carried in the depressed portions of the pattern surface. Exemplary of this system are the techniques disclosed by H. G. Greig in U.S. Pat. No. 2,811,465.
A further technique for developing electrostatic latent images is the liquid development process developed by R. W. Gundlach in U.S. Pat. No. 3,084,043. In this method an electrostatic image is developed or made visible by presenting to the imaging surface a liquid developer from the surface of a developer dispensing member having the plurality of raised portions or "lands" defining a substantially regular surface and a plurality of portions depressed below the raised portions or "valleys." The depressed portions contain a layer of conductive liquid developer which is maintained out of contact with the electrostatographic imaging surface. The development system disclosed in U.S. Pat. No. 3,084,043 hereinafter referred to as the "polar liquid development" system, differs from conventional electrophoretic liquid development systems where substantial contact between liquid developer and both the charged and uncharged areas of the electrostatic latent image occurs. Unlike electrophoretic development, substantial contact between the polar liquid and the areas of the electrostatic latent image bearing surface not to be developed is preferably prevented in the polar liquid development technique. Reduced contact between liquid developer and the nonimage areas of the surface to be developed is desirable because the formation of background deposits is thereby inhibited. Another characteristic which distinguishes the polar liquid development technique from electrophoretic processes is the fact that the liquid phase of a developer actually takes part in the development of a surface. The liquid phase of an electrophoretic liquid developer functions only as a carrier medium for the developer particles.
An additional development technique is that referred to as "wetting development" described in U.S. Pat. No. 3,285,741. In this technique an aqueous developer uniformly contacts the entire imaging surface and due to the selected wetting and electrical properties of the developer substantially only the charged areas of the imaging surface are wetted by the developer.
All these systems have demonstrated good capability in producing developed copies of satisfactory quality. However, the individual systems when employed as commercial embodiments suffer serious deficiencies and drawbacks. In the dry development systems relatively large, bulky, complicated mechanical devices must be employed to effectively accomplish development since the developer materials are presented to the surface in excess quantities which must be removed and the developer materials are also recycled for subsequent development operations. Also, in some of these devices the developer material generally contacts the entire image bearing surface with resulting background deposits on the developed image. Furthermore, the relative free mobility of developer material in such devices yields rather untidy development systems and the possibility of chemical contamination of copy paper, photoreceptor or some mechanical means is always present.
Similar problems often exist in devices employing liquid development systems. Here also the entire image bearing surface may be contacted by the liquid developer increasing the possibility of undesirable background deposits and contributing to the use of excess quantities of developer. Similarly the relatively uncontrolled movement of liquid developer in the device contributes to waste and spillage of developer with the ever attendent possibility of chemical attack of some mechanical part. A particularly significant effect of the unconstrained movement of liquid developer may be contamination of the copy paper supply.
In either the dry or liquid development systems but particularly in the liquid development systems, the developer applicator surface is usually in continuous contact with a portion of the imaging surface. During periods of intermittent use or nonuse the developer present on the applicator may chemically attack the imaging surface resulting in pitting or other surface discontinuities and in the instance where a binder layer photoconductor is used as the imaging surface the developer may completely destroy a portion of the photoconductor resulting in breakdown of the development system. Furthermore, with any binder layer photoconductor the resin binder on attack by the developer may be present thereafter as a contaminant in the developer. Any attacked or degraded photoconductor residue may also be transferred to the surface of the applicator resulting in a non-uniform applicator surface, or in the case of a patterned surface resulting in deposits in the depressed portions of the applicator surface. In addition, the continuous contact which may be under pressure between applicator surface and photoconductor during prolonged idleness may result in the formation of flat splts on one of the surfaces.
Particular difficulty in those systems using a single use or reusable photoconductor is the fact that the entire charged portion of the photoconductor, which may be larger that the image portion, is developed and any copy resulting therefrom may contain deposited or transferred developer in other than image areas.
It is, therefore, clear that there is a continuing need for a better system for developing electrostatic latent images.